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<td class="header">&nbsp;&nbsp;Calibration</td>
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<h3>Calibration Operator&nbsp;</h3>
<p>The objective of SAR calibration is to provide imagery in
which the pixel values can be directly related to the radar backscatter
of the scene. Though uncalibrated SAR imagery is sufficient for
qualitative use, calibrated SAR images are essential to quantitative
use of SAR data.</p>
<p>Typical SAR data processing, which produces level 1 images,
does not include radiometric corrections and significant radiometric
bias remains. Therefore, it is necessary to apply the radiometric
correction to SAR images so that the pixel values of the SAR images
truly represent the radar backscatter of the reflecting surface. The
radiometric correction is also necessary for the comparison of SAR
images acquired with different sensors, or acquired from the same
sensor but at different times, in different modes, or processed by
different processors.</p>
<p>This Operator performs different calibrations for ASAR, ERS,
ALOS and Radarsat-2 products deriving the sigma nought images.
Optionally gamma nought and beta nought images can also be created.</p>
<h4>Products Supported</h4>
<ul>
<li>
<p>Sentinel-1 (IW, EW, SM &nbsp;SLC and GRD) are fully
supported</p>
</li>
<li>
<p>ASAR (IMS, IMP, IMM, APP, APS, APM, WSM) are fully
supported</p>
</li>
<li>
<p>ERS 1 &amp; 2 products (SLC, IMP) are fully supported</p>
</li>
<li>&nbsp; Radarsat-2 products are fully supported</li>
  <li>&nbsp; RCM products are fully supported<br>
  </li>
<li>&nbsp; TerraSAR-X/TanDEM-X products</li><li>&nbsp; ALOS 1 and 2 products</li><li>&nbsp; Cosmo-Skymed products</li>
  <li>&nbsp; Kompsat-5 products</li>
  <li>&nbsp; PAZ products</li>
  <li>&nbsp; RISAT-1 products</li>
  <li>&nbsp; Iceye SLC products<br>
  </li>


<li>
<p>Third party SAR missions may&nbsp;not be fully
supported for all modes.&nbsp;<span style="text-decoration: underline;"></span></p>
</li>
</ul>
<h4>Sentinel-1 Calibration</h4>

<p class="MsoNormal">For converting digital pixel values
to radiometrically
calibrated backscatter, all the required information can be found in
the
product. A calibration vector is included as an annotation in the
product
allowing simple conversion of image intensity values into sigma or
gamma nought
values.</p>

<p class="MsoNormal">The objective of SAR calibration is
to provide imagery in
which the pixel values can be directly related to the radar backscatter
of the
scene. To do this, the application output scaling applied by the
processor must
be undone and the desired scaling must be applied. Level-1 products
provide
four calibration Look Up Tables (LUTs) to produce �<sup>0</sup><sub>i</sub>,
&#963;<sup>0</sup><sub>i</sub>
and &#947;<sub>i</sub> or to return to the Digital Number (DN).
The LUTs apply a
range-dependent gain including the absolute calibration constant. For
GRD
products, a constant offset is also applied. </p>
<p><span style="font-size: 10pt;" lang="EN-US">The
radiometric calibration is
applied by the following equation:</span></p><div style="text-align: center;"><img style="width: 129px; height: 53px;" alt="" src="images/s1_calibration1.png"><br></div><p class="MsoNormal"><span style="" lang="EN-GB">where,
depending on the selected LUT,</span></p><div style="text-align: center;"><img style="width: 676px; height: 71px;" alt="" src="images/s1_calibration2.png"><br></div><p class="MsoNormal"><span style="" lang="EN-GB">Bi-linear
interpolation is used for any pixels that fall between points in
the
LUT.</span></p>
<h4>ASAR Calibration</h4>
<p> For ground range detected products, the following corrections
are applied:</p>
<ul>
<li>
<p>incidence angle</p>
</li>
<li>
<p>absolute calibration constant</p>
</li>
</ul>
<p>In the event that the antenna pattern used to process an ASAR
product is superseded, the operator&nbsp;removes the original
antenna pattern, and apply a new, updated one. The old&nbsp;antenna
pattern gain data is obtained from the external XCA file specified in
the metadata of the source product. The new&nbsp;antenna pattern
gain data and the&nbsp;calibration constant are obtained from the
user specified XCA file. For XCA file selection, user has the following
options:</p>
<ul>
<li>latest auxiliary file (the most recent XCA file available
in the local repository)</li>
<li>product auxiliary file (the XCA file specified in the
product metadata)</li>
<li>external auxiliary file (user provided XCA file)</li>
</ul>
<p>If "product auxiliary file" is selected, then no
retro-calibration is performed, i.e. no antenna pattern gain is removed
or applied. By default the latest XCA file available for the
product&nbsp;is used. <span style="font-weight: bold;"></span></p>
<p>
</p>
<p>For slant range complex products, the following corrections
are applied:</p>
<ul>
<li>
<p>incidence angle</p>
</li>
<li>
<p>absolute calibration constant</p>
</li>
<li>
<p>range spreading loss</p>
</li>
<li>
<p>antenna pattern gain</p>
</li>
</ul>
<p>The&nbsp;antenna pattern gain data and
the&nbsp;calibration constant are obtained from the user specified
XCA file. For XCA file selection, user has the following options:</p>
<ul>
<li>latest auxiliary file (the most recent XCA file available
in the local repository)</li>
<li>external auxiliary file (user provided XCA file)</li>
</ul>
<p>By default, the latest auxiliary file available for the
product will be used for the calibration.</p>
<p></p>
<p> The default output of calibration is sigma0 image. User can
also select&nbsp;gamma0 and beta0 images outputting as virtual
bands in the target product.</p>
<br>
<p>In the following, the calibration process is related to the
different type of ASAR products.</p>
<h4 style="font-style: italic;"> <small><font size="3"> <small>IMS Products</small> </font></small>
</h4>
<p>The sigma nought image can be derived from ESA&#8217;s ASAR level 1
IMS products as the follows (adapted from [1]):</p>
<br>
<img style="width: 650px; height: 190px;" alt="" src="images/calibration_eq1.jpg"><br>
&nbsp;&nbsp;&nbsp; where
<ul>
<li><span style="font-style: italic;">DN</span><sub style="font-style: italic;">i,j</sub><sup style="font-style: italic;">2</sup> is the pixel
intensity for pixel i, j </li>
<li><span style="font-style: italic;">K</span>
is the absolute calibration constant<span style="font-style: italic;"></span></li>
<li><span style="font-style: italic;">&#945;</span><sub style="font-style: italic;">i,j</sub> is incidence
angle </li>
<li><span style="font-style: italic;">R</span><sub style="font-style: italic;">i,j</sub> is the slant
range distance </li>
<li><span style="font-style: italic;">R</span><sub style="font-style: italic;">ref</sub><span style="font-style: italic;"> </span>is the reference
slant range distance </li>
<li><span style="font-style: italic;">&#952;</span><sub style="font-style: italic;">i,j</sub> is the look angle</li>
<li><span style="font-style: italic;">G</span>
is the antenna pattern gain</li>
</ul>
<h4 style="font-style: italic;"> <small><font size="3"> <small>APS Products</small> </font></small>
</h4>
<p>The methodology to derive the sigma nought is the same of the
IMS data but an additional factor (<span style="font-style: italic;">R</span><i ,j=""> / R<sub>ref</sub></i>) must be
taken into account:</p>
<i ,j=""><img style="width: 700px; height: 190px;" alt="" src="images/calibration_eq2.jpg">
<br>
</i>
<h4> <small><i ,j=""><font size="3">
<small>IMP, APP, IMM, APM, WSM Products</small> </font></i></small>
</h4>
<i ,j="">
</i>
<p>In contrast to IMS and APS products, ASAR ground range
imageries (IMP, APP, IMM, APM, WSM) have all been applied antenna gain
pattern compensation (based on ellipsoid approximations) and range
spreading loss correction during the formation of the images. Therefore
the antenna pattern correction applied to the image must be removed
when updated external XCA file is available for the product.
</p>
<p>The sigma nought image can be derived from ESA&#8217;s ASAR level 1
IMP, APP, IMM, APM, WSM products as the follows (adapted from [1]):</p>
<i ,j=""><img style="width: 650px; height: 190px;" alt="" src="images/calibration_eq3.jpg"><br>
</i>&nbsp;&nbsp;For
detailed ASAR calibration algorithm, reader is referred to [1].
<h4>ERS Calibration</h4>
<p>The operator is able to calibrate ERS VMP, ERS PGS CEOS and
ERS PGS ENVISAT ESA standard products generated by different ESA
Processing and Archiving Facilities, such as the German PAF (D-PAF),
the Italian PAF (I-PAF) and the United-Kingdom PAF (UK-PAF), and at the
acquisitions stations such as PDHS-K (Kiruna) and PDHS-E
(Esrin).&nbsp;</p>
<p>For ERS-1 ground range product, the
following&nbsp;corrections are applied:</p>
<ul>
<li>
<p>incidence angle</p>
</li>
<li>
<p>calibration constant</p>
</li>
<li>
<p>replica pulse power variations</p>
</li>
<li>
<p>analogue to digital converter non-linearity</p>
</li>
</ul>
<p>For ERS-1 slant range product, the
following&nbsp;corrections are applied:</p>
<ul>
<li>
<p>incidence angle</p>
</li>
<li>
<p>calibration constant</p>
</li>
<li>
<p>analogue to digital converter non-linearity</p>
</li>
<li>
<p>antenna elevation pattern</p>
</li>
<li>
<p>range spreading loss</p>
</li>
</ul>
<p>For ERS-2 ground range product, the
following&nbsp;corrections are applied:</p>
<ul>
<li>
<p>incidence angle</p>
</li>
<li>
<p>calibration constant</p>
</li>
<li>
<p>analogue to digital converter non-linearity</p>
</li>
</ul>
<p>For ERS-2 slant range product, the
following&nbsp;corrections are applied:</p>
<ul>
<li>
<p>incidence angle</p>
</li>
<li>
<p>calibration constant</p>
</li>
<li>
<p>analogue to digital converter non-linearity</p>
</li>
<li>
<p>antenna elevation pattern</p>
</li>
<li>
<p>range spreading loss</p>
</li>
</ul>
&nbsp;&nbsp;For detailed ERS product calibration algorithm,
reader is referred to [2].
<h4>ALOS PALSAR Calibration</h4>
<p> The operator performs absolute radiometric calibration for
ALOS PALSAR level 1.1 or 1.5 products. For ALOS PALSAR L1.1 and L1.5
products, the following corrections have&nbsp;already been applied:
</p>
<ul>
<li>
<p>range spreading loss correction</p>
</li>
<li>
<p>antenna pattern gain correction</p>
</li>
<li>
<p>incidence angle correction</p>
</li>
</ul>
<p> Therefore the operator applies only the absolute calibration
constant correction to the products.&nbsp;&nbsp;</p>
<p>For detailed ALOS PALSAR product calibration algorithm, reader
is referred to [3].</p>
<h4>Radarsat-2 Calibration</h4>
<p> The operator performs absolute radiometric calibration for
Radarsat 2 products by applying the sigma0, beta0 and gamma0 look up
tables provided in the product.</p>
<p>For detailed Radarsat-2 product calibration algorithm, reader
is referred to [4].</p>
<h4>TerraSAR-X</h4>
<p>The operator performs absolute radiometric calibration for
TerraSAR-X products. The following corrections have been appllied:</p>
<ul>
<li>
<p>calibration constant correction</p>
</li>
<li>
<p>incidence angle correction</p>
</li>
<li>
<p>Noise Equivalent Beta Naught (NEBN) correction</p>
</li>
</ul>
<p>The "noiseCorrectedFlag" in the metadata is checked before
NEBN correction is applied. The operator currently does not support
ScanSAR calibration when noiseCorrectedFlag is false. For detailed
TerraSAR-X product calibration algorithm, reader is referred to [5].</p>
<h4>Cosmo-SkyMed</h4>
<p>The operator performs absolute radiometric calibration for
Cosmo-SkyMed products by applying few product factor corrections.</p>
<p>For detailed Cosmo-SkyMed product calibration algorithm,
reader is referred to [6].</p>
<h4>Kompsat-5</h4>

<p>Currently only High Resolution and Standard Modes are supported by the operator. Wide Swath Modes are not supported. <br>
</p>
<p>High Resolution modes include the following imaging modes:</p>
<ul>
  <li>HR: High Resolution Mode</li>
  <li>EH: Enhanced High Resolution Mode</li>
  <li>UH: Ultra High Resolution Mode</li>
</ul>

<p>Standard Modes include the following imaging modes:</p>
<ul>
  <li>ST: Standard Mode</li>
  <li>ES: Enhanced Standard Mode<br>
  </li>
</ul>

<p>For High Resolution modes, the following radiometric calibration is applied: <br>
</p>
<p><img style="width: 531px; height: 94px;" alt="" src="images/calibration_eq4.jpg"></p>
<p>where <span style="font-style: italic;">F</span> is the rescaling factor and <span style="font-style: italic;">K</span> is the calibration constant in HDF attributes, and <span style="font-style: italic;">&#952;</span><small><span style="font-style: italic;">ref</span> </small>is the reference incidence angle.<br>
</p>
<p>For Standard Modes, the following equation is applied: <br>
</p>
<p><img style="width: 569px; height: 92px;" alt="" src="images/calibration_eq5.jpg"></p>
<p>where &#916;A is the pixel area given by (azimuth spacing x range spacing).<br>
</p>
<h4>Parameters Used</h4>

<p> The parameters used by the operator are as follows:
</p>
<ol>
<li>Source Band: All bands (real or virtual) of the source
product. User can select one or more bands for calibration. If no bands
are selected, then by default, all bands are used for calibration. The
operator is able to detect the right input band. </li>
<li>Auxiliary File: User selected XCA file for antenna pattern
correction. The following options are available: Latest Auxiliary File,
Product Auxiliary File (for detected product only) and External
Auxiliary File. By default,&nbsp;the Latest Auxiliary
File&nbsp;is used. </li>
<li>Scale in dB: Checkbox indicating that the calibrated
product is saved in dB scale. If&nbsp;not checkmarked, then the
product is saved in linear scale. </li>
<li>Create gamma0 virtual band: Checkbox indicating that gamma0
image is created as a virtual band.&nbsp;If&nbsp;not
checkmarked, no gamma0 image is created. </li>
<li>Create beta0 virtual band: Checkbox indicating that beta0
image is created as a virtual band.&nbsp;If&nbsp;not
checkmarked, no beta0 image is created. </li>
</ol>
<p><img style="width: 500px; height: 500px;" alt="" src="images/calibrator_dlg.jpg"></p>
<p></p>
<p style="font-weight: bold;">Reference:</p>
<p>[1] Rosich B., Meadows P., Absolute calibration of ASAR Level
1 products, ESA/ESRIN, ENVI-CLVL-EOPG-TN-03-0010, Issue
1,&nbsp;Revision 5, October 2004</p>
<p>[2]&nbsp;Laur H., Bally P., Meadows P., S�nchez J.,
Sch�ttler B., Lopinto E. &amp; Esteban D., ERS SAR Calibration:
Derivation of &#963;0 in ESA ERS SAR PRI Products, ESA/ESRIN,
ES-TN-RS-PM-HL09, Issue 2, Rev. 5f, November 2004</p>
<p>[3]&nbsp;Lavalle M., Absolute Radiometric and Polarimetric
Calibration of ALOS PALSAR Products, Issue 1, Revision 2, 01/04/2008
</p>
<p>[4] RADARSAT Data Product Specification, RSI-GS-026, Revision
3, May 8, 2000</p>
<p>[5] Radiometric Calibration of TerraSAR-X data -
TSXX-ITD-TN-0049-radiometric_calculations_I1.00.doc, 2008</p>
<p>[6] For further details about Cosmo-SkyMed calibration please
contact Cosmo-SkyMed Help Desk at&nbsp;<font color="black" face="Arial" size="2"><span style="font-size: 10pt; font-family: Arial;" lang="EN-GB"><a href="mailto:info.cosmo@e-geos.it" moz-do-not-send="true">info.cosmo@e-geos.it</a></span></font></p>
<p></p>
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